There are three opportunities to improve AccumulatorTopBottomN<sense, single>::AccumulatorTopBottomN().

We can reserve vector elements so that we don't reallocate it as the vector size grows.

https://github.com/10gen/mongo/blob/master/src/mongo/db/pipeline/accumulator_multi.cpp#L425

We can move objects in follow two cases.

https://github.com/10gen/mongo/blob/master/src/mongo/db/pipeline/accumulator_multi.cpp#L437

https://github.com/10gen/mongo/blob/master/src/mongo/db/pipeline/accumulator_multi.cpp#L442

Better to save this for new engineers and so added neweng label.